1. Field of the Invention
The present invention realtes to a frequency doubler and a visible laser source using the same which is used in the fields of optical information processing using coherent light, optical applied measurement control and the like.
2. Description of Related Art
FIG. 11 is a drawing showing the structure of a frequency doubler of related art. The generation of a harmonic wave (wavelength: 0.41 .mu.m) relative to a fundamental wave having a wavelength of 0.82 .mu.m or less is described in detail below with reference to the drawing (refer to E. J. Lim, M. M. Fejer, R. L. Byer and W. J. Kozlovxky, "Blue Light generation by frequency doubling in periodically-poled lithium niobate channel waveguide", Electronics Letters, Vol. 27, P731-732. 1989). As shown in FIG. 11, a waveguide 2 is formed in an LiNbO.sub.3 substrate 1, and a layer 3 (domain inverted regions) where the domain is periodically inverted is formed across the waveguide 2. The mismatching between the propagation constants of a fundamental wave P1 and a harmonic wave P2 generated can thus be corrected by a periodic structure formed by the domain inverted regions 3 and domain non-inverted regions 5, thereby generating a harmonic wave P1 with a high degree of efficiency from the fundamental wave P1 incident upon an incidence surface 10.
The principle of harmonic amplification is described below with reference to FIG. 12. A domain non-inverted element 31 in which the domain are inverted has no domain inverted regions and thus has a single domain direction. The harmonic output power 31a of the domain non-inverted element 31 merely increases and decreases in repetition. However, the harmonic output power 32a of a domain inverted frequency doubler (primary period) 32 in which the domains are periodically inverted increases in proportion to the square of the length l of the waveguide formed in the element, as shown in the drawing.
However, through the domain inversion, the output power of the harmonic wave P2 relative to the fundamental wave P1 cannot be obtained until quasi phase-matching is established. The establishment of quasi phase-matching is limited to such a case that the periods .LAMBDA.1 (shown in FIG. 12) of the domain inverted regions are .lambda./(2(N2.omega.-N.omega.)) wherein N.omega. is the effective refractive index of the fundamental wave (wavelength .lambda.) and N2.omega. is the effective refractive index of the harmonic wave (wavelength .lambda./2). The above-described frequency doubler of related art has a domain inverted structure as a base.
The method of producing the doubler is described below with reference to FIGS. 13a to 13c. As shown in FIG. 13a, a Ti pattern 31 is formed at intervals of several .mu.m on an LiNbO.sub.3 substrate 1 consisting of a non-linear optical material by lift-off and evaporation. As shown in FIG. 13b, domain inverted regions 3 in which the domain are inverted to the direction opposite to that of the LiNbO.sub.3 substrate 1 are formed by heat treatment at a temperature of about 1100.degree. C. As shown in FIG. 13c, a waveguide 2 is then formed by heat treatment in benzoic acid (at 200.degree. C.) for 20 minutes and then annealing at 350.degree. C. for 3 hours. The frequency doubler produced by the treatment with benzoic acid has the waveguide having a length 1 mm for the fundamental wave P1 having a wavelength of 820 nm and generates the harmonic wave P2 with an output power of 940 nW when the output power of the fundamental wave P1 is 14.7 mW.
In the above-described frequency doubler having as a fundamental component the domain inverted regions, the tolerance for variations of the wavelength of the fundamental wave is as small as 0.1 nm in terms of half band width when the length of the element is 5 mm. Namely, if the laser wavelength is changed by 0.1 nm, the output power is halved. The combination of the frequency doubler with a semiconductor laser diode therefore has the problem that when the laser wavelength of the semiconductor laser diode is changed with a change in the temperature thereof, no harmonic wave is generated or the harmonic output power is significantly changed. This problem is described in detail below.
FIG. 14 shows the relation between a change in the laser wavelength of a semiconductor laser diode and the harmonic output power when the environmental temperature is changed. As shown in FIG. 14, although the harmonic output power is highest at a wavelength of 820 nm, the harmonic output power is halved if the laser wavelength deviates by only 0.05 nm. The tolerance for changes in the laser wavelength of the semiconductor laser diode is thus very small. When the environmental temperature is changed from 20.degree. C. to 21.degree. C., since the oscillation wavelength of the semiconductor laser diode is changed by 0.2 nm from 820 nm to 820.2 nm, the harmonic output power becomes zero. The above frequency doubler thus has such a fault that the harmonic output power is significantly affected by changes in the environmental temperature.